Cytochrome P450 2E1 and 2A6 enzymes as major catalysts for metabolic activation of N-nitrosodialkylamines and tobacco-related nitrosamines in human liver microsomes

Abstract

An acetyltransferase-overexpressing strain of Salmonella typhimurium (NM2009) has been used to investigate roles of human liver microsomal cytochrome P450 (P450) enzymes in the activation of carcinogenic nitrosamine derivatives, including N-nitrosodialkylamines and tobacco-smoke-related nitrosamines, to genotoxic products. Studies employing correlation of activities with several P450-dependent monooxygenase reactions in different human liver samples, inhibition of microsomal activities by antibodies raised against human P450 enzymes and by specific P450 inhibitors, and reconstitution of activities with purified P450 enzymes suggest that the tobacco-smoke-related nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and N-nitrosonornicotine (NNN) as well as N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) are oxidized to genotoxic products by different P450 enzymes, particularly P450 2E1 and 2A6. The activation of NDMA and NNN by liver microsomes was suggested to be catalyzed more actively by P450 2E1 than by other P450 enzymes because the activities were well correlated with NDMA N-demethylation and aniline p-hydroxylation in different human samples, and purified P450 2E1 had the highest activities in reconstituted monooxygenase systems. The relatively high contribution of P450 2A6 to the activation of NDEA and NNK was supported by the correlation seen with coumarin 7-hydroxylation in human liver microsomes, and antibodies raised against P450 2A6 inhibited both activities by approximately 50%. P450 3A4, 2D6 and 2C enzymes appear not to be extensively involved in the activation of these nitrosamines as judged by several criteria examined. Thus, this work indicates that several P450 enzymes, particularly P450 2E1 and 2A6, catalyze metabolic activation of nitrosamine derivatives including N-nitrosodialkylamines and tobacco-smoke-related nitrosamines in human liver microsomes.

Human Cytochrome P450 2E1 Mutations That Alter Mitochondrial Targeting Efficiency and Susceptibility to Ethanol-induced Toxicity in Cellular Models

“…CYP2E1 antibody (50 g), SKF 525-A (1 mM), and disulfiram (25 M) were also used as controls to ascertain the role of CYP2E1. Details were essentially similar to those for the erythromycin N-demethylation assay published previously (28,63,64).…”

A novel CYP2A6 allele, CYP2A6*23, impairs enzyme function in vitro and in vivo and decreases smoking in a population of Black-African descent

“…Cotinine is further metabolized to trans-3′-hydroxycotinine and this reaction is entirely mediated by CYP2A6 [4]. CYP2A6 is also the main enzyme metabolizing therapeutic compounds such as coumarin [5], tegafur [6], and SM-12502 [7], and it can bioactivate tobacco smoke procarcinogens such as N′-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) [8].…”

Chemoprevention of bicyclol against hepatic preneoplastic lesions

“…28 Therefore, these data suggest that CYP2B1 is presumedly as one of the major Bicyclol-induced P450 isoform(s) that attributes to the denitrosation of DEN in rat hepatic microsome, but the total spectrum of P450 isoforms after Bicyclol induction differs from the CYP2B1-inducer phenobarbital. So, further study requires using CYP isoform specific antibodies and isoform-specific blockers to clarify if the specific CYP isoform(s) otherwise CYP2B1, like CYP2A6 in previously reported, 59 is correlated to the Bicyclol-induced increase in denitrosation of DEN.…”

“…At least three CYP families: 2B, 2A and 2E have been reported as the major CYP genes involved in biotransformation of dialkylnitrosamines, [58][59][60] and are presumedly correlated to Bicyclol inducible P450 activities for DEN metabolism. Shu and Hollenberg indicated that CYP2E1 was one of the P450 isoforms for short chain substrates such as DMN.…”